1. Field of the Invention
Embodiments of the present invention relate to a semiconductor package formed of multiple semiconductor die stacked on substrate layers in a three dimensional arrangement.
2. Description of the Related Art
The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices. These devices include for example digital music players, cellular phones, handheld PCs, digital cameras, digital video camcorders, smart phones, car navigation systems and electronic books.
Flash memory storage cards come in a number of different configurations, but generally include a semiconductor package housed within a standard sized and shaped enclosure. These standard enclosures include SD (Secure Digital) cards, Compact Flash, a Smart Media, a Mini SD Card, an MMC, an xD Card, a Transflash memory card or a Memory Stick. The semiconductor package used in such memory devices includes an integrated circuit typically having passive components, one or more memory chips and, in some configurations, a controller chip mounted on and electrically connected to a substrate. Substrates on which the integrated circuit may be formed include printed circuit boards, leadframes and polyimide tapes. Once formed on the substrate, these integrated circuits are typically encapsulated in a molding compound which protects the integrated circuit and removes heat from the package.
Where once memory devices included a plurality of discrete semiconductor packages, each handling different functions, currently a plurality of integrated circuit components may be packaged together to provide a complete electronic system in a single package. For example, multichip modules (“MCM”) typically include a plurality of chips mounted side by side on a substrate and then packaged. Another example is a system-in-a-package (“SiP”), where a plurality of chips may be stacked on a substrate and then packaged.
With form factors being fixed for most of the standard memory cards currently in use, there are generally only two ways to increase the memory density within a card: use higher density memory chips, and stack more memory die in a single package. As the memory card space is limited, the method of stacking more memory chips in a package is getting more difficult and expensive.
Another concern with fabricating stacked memory die in packages is the stresses to which the stacked die are subjected during the encapsulation process. Encapsulation equipment may output an injection force of about 0.8 tons to drive the molding compound into the mold cavity. For die having a footprint of about 4.5 mm by 2.5 mm, this injection force may result in a pressure down on the die of about 1.2 kgf/mm2. In the past, semiconductor die were better able to withstand the stress generated during the molding process. However, die thicknesses have been reduced to a range of about 2 mils to 13 mils. At these thicknesses, the die are often not able to withstand the stresses generated during the encapsulation process, and fractures in one or more of the die, known as die cracking, may occur.
A further concern with stacked memory die is that die are tested after they are stacked together and mounted on the substrate. If one of the die turns out to be defective, due to die cracking or any other reason, the entire die stack must be discarded.